My guess: the book's organization mirrors a life, having a defining originating event (birth), a defining concluding event (death), and between them a phenomenon unfolding in time, with its recurrences, ragged patterns, and dumb persistence.
The titles of the individual poems also all begin or end with the letter "C," although an initial or terminal "c" in a poem's title does not entail its inclusion in the section with the appropriate name -- too obvious, perhaps.
I invested a little time in the hypothesis that the poems in "Initial C" were about beginnings and those in "Terminal C" about endings, but I had to give up on that idea -- likewise too obvious, I suspect. However, the poems in the first section do have an airier, lighter quality, like a water-color painting, are more frequently set outdoors, seem more hopeful, while those in the final section are denser, more tangled, angrier. I preferred the poems of the third section to those of the first, but that's just me; there are excellent poems in both.
"Chronic," though, for me, was far and away the highlight of the volume, a poem I expect to revisit frequently. To place a poem titled "chronic" in a section called "Chronic" in the center of a book titled Chronic is to impose upon it a burden of expectation that few poems can fulfill -- but "chronic" is more than equal to the challenge. I've read it six or seven times in the last few days, and I think it belongs in the company of Wordsworth's "Intimations" ode as a meditation on loss, on the marriage of our minds and our bodies to nature and that marriage's inevitable decline and end.
Sometimes the poem deliberately summons an echo of 19th century poetry, as in the syntactical inversion of "and delight I took in the sex of every season" or the Hopkins-like twist of "vibrant arc their swift, their dive against the filmy, the finite air." I even hear a little Yeats ("I carry the same baffled heart I have always carried" -- cf. opening lines of "The Tower"). Or the Wordworthian catalogue of this line, combined with the question of why one feels compelled to make catalogues:
why do I need to say the toads and moor and clouds --
Yet Powell has in some ways more on his plate than Wordworth had -- the lights were going out in Wordsworth's imagination, but he was physically healthy, which Powell is not ("daily I mistake -- there was a medication I forgot to take"), and while Wordsworth did have to worry about the destruction of the English countryside, he did not have to worry whether humans would make the planet uninhabitable:
choose your own adventure: drug failure or organ failure
cataclysmic climate change
or something akin to what's killing bees -- colony collapse
The concluding lines takes a tag-line from the Homeric Hymns and turn it into a heartbreaking plea:
light, light: do not go
I sing you this song and I will sing another as well
Chronic is a fine book -- "chronic" something extraordinary.